Heterogeneous potassium isotopic composition of the upper continental crust

Assessing the K isotopic composition of the upper continental crust is important for understanding the processes by which the crust is generated and modified, and constraining the K isotopic budget of the silicate Earth. High-precision K isotopic data are reported for 70 well-characterized individual and composite samples from the upper continental crust, including diorite, granodiorite, granite, loess, shale, graywacke, pelite, and tillite, to constrain its K isotopic composition. delta K-41 varies significantly in eight I-type and two S-type granites from Australia (-0.57 to -0.40 parts per thousand), nine A-type granites (-0.53 to -0.38 parts per thousand), and three granitoid composites including diorite, granodiorite, and granite (-0.50 to -0.37 parts per thousand) from China, mainly reflecting source heterogeneity. The 22 post-Archean Australian shales (PAAS) (delta K-41 = -0.68 to -0.12 parts per thousand) and the 12 sedimentary composites including graywacke, pelite, and tillite from China (delta K-41 = -0.57 to -0.23 parts per thousand) have heterogeneous K isotopic compositions while the 12 loess samples from around the world display more limited K isotopic variation (delta K-41 = -0.47 to -0.35 parts per thousand). delta K-41 values display a smaller dispersion in loess compared to shales, which have comparatively more intense weathering and higher chemical index of alteration (CIA). delta K-41 correlates with Fe2O3/Al2O3 and Fe2O3/K2O in shales and Al2O3/SiO2, K2O/Al2O3, and delta Li-7 in loess, suggesting that K isotopes are fractionated during chemical weathering. Overall, the upper continental crust has a heterogeneous K isotopic composition, ranging from -0.68 to -0.12 parts per thousand with an average delta K-41 of -0.44 +/- 0.05 parts per thousand (2SD, n = 88), which is indistinguishable from the mantle. (C) 2019 Elsevier Ltd. All rights reserved.